Problem 73 - Chemistry Principles and Practice

Question

Identify the orbitals on each of the atoms that form the bonds in \(\mathrm{H}_{3} \mathrm{CCN}\). How many \(\sigma\) bonds and \(\pi\) bonds form?

Step-by-Step Solution

Answer

5 sigma bonds and 2 pi bonds.

Step 1: Understand the Molecular Structure

The molecule in question is acetylene nitrile (acetonitrile) or \(\mathrm{H}_3\mathrm{CCN}\). Let's break it down: it consists of a carbon atom bonded to three hydrogen atoms (forming methyl group \(\mathrm{CH}_3\)), a carbon atom triple bonded to a nitrogen atom (nitrile group \(\mathrm{CN}\)). The structure can be viewed as \(\mathrm{H}_3\mathrm{C}-\mathrm{C} \equiv \mathrm{N}\).

Step 2: Identify the Hybridization of Atoms

For each atom, determine the hybridization state: the first carbon (\(\mathrm{CH}_3\)) is \(\mathrm{sp}^3\) hybridized, forming \(\sigma\) bonds with three hydrogen atoms and the second carbon. The second carbon attached to nitrogen is \(\mathrm{sp}\) hybridized because it is involved in forming a triple bond with nitrogen. The nitrogen is also \(\mathrm{sp}\) hybridized.

Step 3: Identify the Orbitals Forming the Bonds

In \(\mathrm{H}_3\mathrm{C}-\), each carbon-hydrogen bond involves an \(\mathrm{sp}^3\)-\(\mathrm{s}\) overlap, forming a \(\sigma\) bond. The \(\mathrm{C}-\mathrm{C}\) bond is formed by \(\mathrm{sp}^3\)-\(\mathrm{sp}\) overlap, and the \(\mathrm{C} \equiv \mathrm{N}\) bond includes one \(\sigma\) bond from \(\mathrm{sp}\)-\(\mathrm{sp}\) and two \(\pi\) bonds from the overlap of \(2p-2p\) orbitals (for both \(\mathrm{C}\) and \(\mathrm{N}\)).

Step 4: Count the Sigma and Pi Bonds

Count the number of \(\sigma\) and \(\pi\) bonds: in total, there are 5 \(\sigma\) bonds (three \(\mathrm{C}-\mathrm{H}\) bonds, one \(\mathrm{C}-\mathrm{C}\) bond, and one \(\mathrm{C}-\mathrm{N}\) bond) and 2 \(\pi\) bonds (in the \(\mathrm{C} \equiv \mathrm{N}\) triple bond).

Key Concepts

Hybridization

Hybridization is a concept in chemistry that explains how atomic orbitals mix to form new hybrid orbitals. These are the orbitals that atoms use to make bonds. In the case of acetonitrile (\(\mathrm{H}_3\mathrm{CCN}\)), let's see how each carbon atom is hybridized.

• The first carbon, connected to three hydrogen atoms, uses \(\mathrm{sp}^3\) hybrid orbitals. This accounts for its ability to form four sigma bonds: three with hydrogen and one with the neighboring carbon atom.
• The second carbon, which forms a triple bond with nitrogen, is\(\mathrm{sp}\) hybridized. This means it uses\(\mathrm{sp}\) orbitals to form a sigma bond with the nitrogen and another sigma bond with the first carbon.
• Nitrogen is also\(\mathrm{sp}\) hybridized as it shares the triple bond with carbon, requiring one\(\mathrm{sp}\) orbital for the sigma bond in the triple bond formation.

Hybridization is crucial for understanding molecular geometry and bond formation. It shows how electron distribution and orbital overlap influence how molecules behave and interact.

Sigma Bonds

In simple terms, sigma (\(\sigma\)) bonds are the strongest type of covalent bonds formed from the end-to-end overlap of atomic orbitals. They are characterized by their cylindrical symmetry around the bond axis. In acetonitrile (\(\mathrm{H}_3\mathrm{CCN}\)), several sigma bonds hold the structure together.

• The three\(\mathrm{C}-\mathrm{H}\) bonds are sigma bonds formed by the overlap of \(\mathrm{sp}^3\) hybrid orbitals from carbon with \(\mathrm{s}\) orbitals from hydrogen.
• The bond between the two carbon atoms is another sigma bond formed from the overlap of an\(\mathrm{sp}^3\) orbital from the first carbon and an\(\mathrm{sp}\) orbital from the second carbon.
• The carbon-nitrogen bond also contains a sigma bond formed by the overlap of \(\mathrm{sp}\) orbitals on both carbon and nitrogen atoms.

Altogether, there are five sigma bonds in total, which maintain the molecule's structural integrity by keeping the atoms firmly in place.

Pi Bonds

Pi (\(\pi\)) bonds occur when parallel p orbitals overlap. This creates a bond that is above and below the bond axis, unlike sigma bonds, which are directly between the atoms. Pi bonds are usually found in addition to sigma bonds in double or triple bonds, providing extra bonding strength.

In \(\mathrm{H}_3\mathrm{CCN}\), the carbon-nitrogen triple bond showcases this well.

• The two remaining unhybridized p orbitals on each carbon and nitrogen atom contribute to \(\pi\) bonds by overlapping sideways.
• These two\(\pi\) bonds along with the sigma bond form the triple bond between carbon and nitrogen.

This means there are two pi bonds in the carbon-nitrogen part of acetonitrile. Despite being weaker than sigma bonds individually, pi bonds provide essential strength in a structure like a triple bond, giving it more energy and rigidity.

Molecular Structure

The molecular structure of acetonitrile (\(\mathrm{H}_3\mathrm{CCN}\)) helps determine many of its properties. Structurally, it comprises a methyl (\(\mathrm{CH}_3\)) group and a nitrile (\(\mathrm{CN}\)) group.

• Methyl Group (\(\mathrm{CH}_3\)): Consists of a carbon atom bonded directly to three hydrogen atoms. This part is tetrahedral with \(\mathrm{sp}^3\) hybridization, implying 109.5° bond angles.
• Nitrile Group (\(\mathrm{CN}\)): Comprises a carbon-nitrogen triple bond. Both atoms in this group are \(\mathrm{sp}\) hybridized, making it linear with bond angles of 180°.
• Overall Structure: Acetonitrile balances a tetrahedral configuration on one end with linear on the other, making it interesting in terms of reactivity and solubility characteristics.

Understanding the molecular structure aids in predicting the chemical and physical behavior of acetonitrile. The interplay between different bonds and hybridization shapes gives insight into why it reacts or dissolves under certain conditions.